@gervais if you'd like to send me one the plugs I can examine it with a microscope in the lab, and stain the white material (after removal) for DNA, which would tell you if there are cells in there or if it's just mineral (salt) material.Sorry to tell you, but there are molds (fungi) that have fruiting bodies with a crystalline appearance. Unless you have some training in mycology (which I don't) and the facilities to examine the stuff for spores, you have no idea what it is. You're just guessing.
My wife is the type that throws bread away when a little mold grows on it. I brush it off and toast it and eat it. Mold and other funky things are a natural part of beer and bread, they grow on our skin and in our mouths while we sleep. Fungal, bacterial, or not, activity of this type is not *spoilage* of the tobacco. I have lots of aged tobacco, some 70+ years old, and I can tell you with confidence that this stuff, whatever it is, is a natural and normal part of aging of plug tobacco. If the appearance is off-putting then don't smoke it but we're in you I'd trade or sell it to another less-squeamish smoker rather than throw it away.
For my part, just as I enjoy a nice brie with a mold crust, this is just another interesting feature of a delicacy that happens to be tobacco.